The background of the present invention is described herein in the context of pay television systems, such as cable television and direct broadcast satellite (DBS) systems. However, the invention is by no means limited thereto except as may be expressly set forth in the accompanying claims.
In the pay television industry, programmers produce programs for distribution to various remote locations. A "program" may consist of video, audio, and/or other related services, such as closed-captioning and teletext services. A programmer supplies these services, e.g., via satellite, to individual subscribers and/or cable television operators. Typically, many separately encoded, compressed programs are multiplexed together and sent over a data channel to cable television operators. The cable operators receive the multiplex and select the programs/services they wish to distribute to their subscribers. The selected programs are then transmitted to the individual subscribers via a coaxial cable distribution network.
In the past, pay television systems have operated in the analog domain. Recently, however, the pay television industry has begun to move toward all digital systems wherein, prior to transmission, the analog program signals are converted to digital signals. Digital signal transmission offers the advantage that digital data can be processed at both the transmission and reception ends to improve picture quality. In addition, digital data compression techniques can achieve high signal compression ratios. Digital compression allows a larger number of individual services to be transmitted within a fixed bandwidth. Bandwidth limitations are imposed by both satellite transponders and coaxial cable distribution networks, and therefore digital compression is extremely advantageous.
FIG. 1 depicts an exemplary application of a vector quantization image compression system wherein image data is communicated from a point of origin 12 to receiving locations 14, 16. (It should be noted that the invention is not limited to use in a vector quantization system, or any other particular type of encoding system. For example, the invention may be employed in a system using discrete cosine transform (DCT) encoding, such as an MPEG system.) The point of origin 12 might include a source 20 of program material that supplies movies and the like in analog form to an encoder 22 for digitization and data compression by vector quantization. Compressed digital data is transmitted to a satellite 18 via transmitter 24 for reception by a plurality of earth stations 14, 16. The earth stations 14, 16 may be the head-end of a cable television distribution system of the type which receives signals from the satellite 18 and distributes them to a plurality of subscribers via coaxial cable or optical fiber. Alternatively, one or more of the earth stations may be DBS (direct broadcast satellite) subscribers who receive signals directly from the satellite 18. The cable head-end installation 14 may receive the data transmitted by the station 12 via a receiver 26 and then employ an on-site decoder 28 for decompressing the received data and converting the same back to analog form. Another on-site apparatus 30 may convert the analog data to conventional NTSC signals for transmission over cable to subscribers in conventional form. Thus, in the case of cable head-end installation 14, the cable head-end operator distributes analog NTSC cable television signals to subscribers in conventional form. Further background on digital television can be found in U.S. patent application Ser. No. 968,846, Oct. 30, 1992, titled System and Method for Transmitting a Plurality of Digital Services, and U.S. Patent No. __ (application Ser. No. 794,516, issued Dec. 15, 1992), titled Image Compression Method and Apparatus Employing Distortion Adaptive Tree Search Vector Quantization.
A problem with the prior art is caused by the fact that, when the program provider decides to change the data rate at which a given program is encoded and provided to cable operators, there is often a noticeable interruption in the picture received by the viewers that are watching that program. One reason for effecting such a rate change would be to increase the amount of data received by the cable operators and to thereby improve the quality of the television pictures received by the viewers. Typically, the program multiplex is provided to the data channel at a fixed rate, although the individual programs composing the multiplex are compressed with a variable rate coding scheme. This means that buffers are required at the encoder(s) and decoder(s). If the encoding rate R.sub.i of one program is changed in a system with a fixed rate data channel, the encoding rates of all other programs must be adjusted such that the total data rate R is fixed (where R equals the sum of the data rates of the individual program channels). As programs are encoded by a program provider prior to being sent to a cable operator, the encoding rate for each program must be controlled to avoid decoder buffer overflows and underflows, which, if allowed to occur, would produce a noticeable interruption in the viewers' television pictures.
Accordingly, a primary goal of the present invention is to provide methods and apparatus whereby the encoding rate is adjusted automatically to avoid any overflows or underflows in the viewers' decoder buffers.